Saturday, November 12, 2016

DIY kits 70W SSB linear HF Power Amplifier For YAESU FT-817 KX3

My first impression : High quality PCB ( 100 x 50mm ) , all parts in the kit including 2 power FETs type IRF530N. Capacitors are 10nF instead of 100nF as shown in the schematic but that's not a problem. The 0.5 turns windings are on the PCB by means of the tubes inside the toroids. Construction is easy but a lowpass filter for each band is mandatory. I used the low pass data from W3NQN for a 7 element Chebyshev with standard value silver mica capacitors and T68-6 toroids. The provided wire for the power chocke ( 2 turns ) has a weak coating . It's better to replace it to avoid shortcuts .Start soldering the small parts and end with the transformers. Turn the potentiometer anti clockwise and slowly adjust the bias to aprox. 3.7V. I replaced the FETs by a pair of IOR IRF530N resulting in more stabillety en less input SWR however the input swr is highly effected by the drive power . At 5 Watt the swr mounts to over 1:3... . Best results with this PA are obtained on CW and FM .
The test was in the original configuration at 13.8V and 5Watt FM input from my FT-817. Max. power was 80 Watt . At 3 Watt input the output was about 50 Watt at 20m. but the modulation was as expected distorded in ssb mode at the recommended 2.7V bias voltage.
At 3.7V ( gate ) bias the modulation is acceptable but far from perfect. The 150 Ohm resistors over the FETs heat up strongly at full power FM. It's better to reduce the drive power to 2 Watt when working 12V FM/AM . Don't reduce the drive by reducing the Bias voltage for this results in a distorted modulation in SSB mode. Better use a power attenuator : Power att. calculator PA1B
Unfortionally the seller often forgets to add the documentation, the picture shows a copy of the original pdf file.
A You Tube impression by KC2IRV : Overall impression of the amplifierTesting the PA
Visit the blog of John DK9JC for a nice test : Link

Conclusion : This is a nice kit with acceptable results for a low price. For best results reduce the drive power to 2.5 Watt.
swr at 5 Watt input : 1:3+. swr at 2.5 Watt input 1:1.8 with more than 50 Watt output. This is more than enough for fieldwork.

copy of the original pdf file

A second amplifier was build without cooling fan . You can clearly see the 7 element filter for 20m